This invention relates to ethylene vinyl alcohol copolymer compositions which exhibit improved resistance to oxidative and thermal degradation and gelation.
Ethylene vinyl alcohol copolymer (EVOH) as formed may not have adequate thermal or oxidative stability for certain applications. It is often observed that the melt viscosity of EVOH resins increases with time, and the rate of increase increases with temperature. It is presumed that this effect results from the increasing strength of residual acid from the washing process, at higher temperatures. It is known that acids catalyze crosslinking reactions in EVOH, strong mineral acids such as HCl being especially potent. It is also possible that spontaneous dehydroxylation occurs at elevated temperatures, leading to interchain linkages. The resulting increase in viscosity can eventually cause gelation or solidification of the molten polymer. Such behavior can result in metal surfaces of processing equipment becoming coated with a varnish-like layer of intractable, discolored, degraded polymer. Buildup of this layer can cause a gradual rise in torque required for extruder screws and, when exfoliated, the sporadic appearance of gel particles in the extruded product. Furthermore, when molten EVOH is exposed to oxygen, e.g. through air incursion at feed and vent ports of extruders, the polymer rapidly darkens and crosslinks to yellow-brown gel. This gelation can occur in only 10-20 minutes. As a result of these problems, comparatively low melt processing temperatures (210.degree.-230.degree. C.) are normally recommended for processing EVOH polymers, and even then such problems can persist.
There has been much activity to find a way to increase the thermal and oxidative stability of EVOH. South African patent application 86/07151 (British appln. 2,182,334) discloses a vessel comprising a composition of EVOH and a propylene resin or a thermoplastic resin having in the main or side chain a carbonyl group. The resin mixture is stabilized against gelation, discoloration, and reduction of gas barrier properties by incorporation of a hydroxide or higher fatty acid salt of an alkaline earth metal and an antioxidant. The higher fatty acid salts include barium, calcium, or especially magnesium salts of saturated or unsaturated fatty acids having 10-26 carbon atoms. There is no disclosure of salts of fatty acids having fewer than 10 carbon atoms. Sterically hindered phenols are preferred antioxidants.
U.S. Pat. No. 4,704,423 discloses molded articles comprising a mixture of an olefin resin which includes residual chlorine due to a Ziegler type catalyst, a hydrolyzed ethylene-vinyl acetate copolymer, and a hydrotalcite compound, exhibiting reduced gel formation during processing. The composition optionally contains a metal salt of a higher fatty acid, especially stearic acid, and usual additives such as a stabilizer. The higher fatty acids are those of 12 or more carbon atoms; there is no disclosure of salts of lower molecular weight acids nor of combainations of such salts with antioxidants.
Japanese patent application 64-69653 (and its cognate 01-069653) and European application 0 308 703 disclose an EVOH composition containing 0.005-0.05 wt % of a salt of a group II metal, 0.002-0.2 wt. % of an acidic substance with a pKa value of 3.5 or above and a boiling point of 180.degree. C. or above, 0.01-0.2 wt % of an acidic substance with a pKa value of 3.5 or above and a boiling point of 120.degree. C. or below and 0.01-1 wt. % of a hindered phenol containing ester. The high boiling acid can be caprylic acid; the low boiling acid can be formic, acetoacetic, or preferably acetic acid. The salts include beryllium, magnesium, calcium, zinc, and barium--carbonates, sulfates, phosphates, chlorides, or preferably acetates. All the examples appear to include acetic acid or salts thereof as a component, and there is no teaching that metal salts of the higher boiling acids be used. The composition yields a molded product which is unaccompanied by gelation.
Japanese application 62-143954 discloses saponified ethylene vinyl acetate copolymer treated with an acid, preferably acetic acid/H.sub.3 PO.sub.4, and a calcium or magnesium salt of a polybasic acid, preferably phosphoric acid. The treated copolymer has high heat stability.
Japanese application 54-87783 discloses a method to improve the adhesive property between a layer of polyolefin (grafted with acid or anhydride) laminated to and a layer of ethylene-vinyl alcohol copolymer, a polyamide resin, a polyester resin, or a hydrocarbon synthetic elastomer. A phenol antioxidant and a higher fatty acid salt having 7-23 carbon atoms, particularly calcium laurate, palmitate, or stearate are added to the polyolefin layer or to both layers. The examples show the salts present in polyolefin layer.
Japanese application 63-286459 discloses a composition of an ethylene vinyl alcohol copolymer with an amide having a hindered phenol group. An exemplified composition took 75 minutes until the rate of torque rise amounted to 1.5 kg-cm/min when kneaded at 220.degree. and 90 r.p.m. under air, compared with 6 minutes for a composition containing instead BHT. There is no disclosure of the use of salts of certain aliphatic carboxylic acids.
U.S. Pat. No. 3,926,876 discloses a flame-resistant resinous molding of ethylene-vinyl alcohol copolymer, 5 to 50 parts of a haloorganocompound, 0.2 to 10 parts of a metallic soap, and 0 to 30 parts of antimony trioxide. The metallic soaps are metal salts of fatty acid other than alkali metal salts, such as certain stearates, laurates, and ricinoleates. There is no disclosure of the addition of hindered phenolic antioxidants nor salt of lower acids.
U.S. Pat. No. 4,753,760 discloses a method of preparing saponified product pellets of ethylene-vinyl acetate copolymer extruding a solution of the copolymer in methanol or water-methanol mixture containing a lubricant into strands which are cut and pelletized. The lubricant can be e.g. ethylene bis-stearic amide or a fatty acid metallic salt such as potassium stearate. There is no disclosure of addition of salts of aliphatic carboxylic acids having 3-9 carbon atoms nor of antioxidants.